1. Field of the Invention
The present invention generally relates to flat antenna devices and, more particularly, to a flat antenna device applicable to communication, a radar apparatus, etc which is light and which is not easily deformed due to a temperature change.
2. Description of the Related Art
FIG. 11 shows a prior art flat antenna device disclosed, for instance, in Japanese Laid-Open Patent Application No. 60-10805. Reference numeral 8 indicates a radiation element, numeral 9 indicates a honeycomb dielectric core, numeral 10 indicates a grounding conductor, and numerals 11a and 11b indicate dielectric skins. In this antenna device, a microstrip patch antenna is formed by sandwiching the honeycomb dielectric core 9 and the dielectric skins 11a and 11b between the rectangular radiation element 8 and the grounding conductor 10.
When the weight of the prior art flat antenna device as constructed above is to be reduced, the honeycomb dielectric core 9 may be enlarged or the dielectric skins 11a and 11b may be made thin. When the weight of the flat antenna device is reduced by enlarging the honeycomb dielectric core 9, there is a problem in that the flatness of the flat antenna device is deteriorated due to the bending of the dielectric skins 11a and 11b, resulting in deterioration in the electric performance of the device.
The weight of the flat antenna device may also be reduced by making the dielectric skins 11a and 11b thin. However, in order to maintain the strength of the device, the thickness of the dielectric skins 11a and 11b may be decreased only to a certain degree. Therefore, reduction in the weight of the device according to such a method is effective only to a certain extent.
It is to be noted that the flat antenna device as described above may be used in an environment where a significant temperature change is caused. For example, the device may be used in a satellite orbit. In general, the coefficient of thermal expansion of metal members fitted to the grounding conductor sheet or the radiation element sheet differs significantly from that of the other sheets. This produces a problem that, if the antenna device is used in an environment where a significant temperature change occurs, the flatness of the antenna suffers due to bimetal deformation, resulting in deterioration in the electric performance of the antenna device.
Moreover, the metal member used to form the grounding conductor and the radiation element is larger in specific gravity than the material for the other sheets, thus making it even more difficult to reduce the weight of the flat antenna device.